How to Defend Earth Against an Asteroid Strike

In troubled economic times, it’s often hard to convince the government to fund space science. Heck, at least those much-studied fruit flies live on our planet. But there’s one field of research that the public should be happy to support: keeping the Earth from being pummeled by asteroids. And there is no shortage of ideas for how to do this.

Then there’s an asteroid spotted in 2004 and called Apophis. Astronomers originally thought it might hit Earth in 2029. Then they decided that it couldn’t. Finally they moved back the clock to 2036.

The uncertainty is understandable, but not exactly reassuring. And even if Apophis misses, some other rock big enough to put a serious dent in Earth and everything living here will take dead aim for us someday. It’s just a matter of time. Some researchers put the odds of a civilization-wrecker at one in the next 300,000 years, others at 1 in 10 for the next century.

But when our luck finally runs out, humanity will have something even more useful: guns. As described in the scheduled proceedings of the upcoming first International Academy of Astronautics Planetary Defense conference, engineers have come up with plenty of ways to nudge an Earth-bound asteroid off-course, or failing that, obliterate it from its existence. Here are some of their ideas.

Direct nuclear explosions. As immortalized in the movie Armageddon, we could blow an asteroid out of the sky like so much interplanetary skeet. But there’s a catch: The pieces could still hit Earth, and we might not have enough firepower to do serious damage. It’s a last-minute, last-ditch option.

Nearby nuclear explosions. Rather than blowing it up, a nuclear explosion could reroute a space rock’s trajectory clear of Earth.
This would need to be done decades before the asteroid reached us. They are, after all, not easy to steer. But models suggest it could work.

Laser sublimation. Several spacecraft could use machines that direct beams of concentrated sunlight to the surface of an appropriately icy asteroid. As it heats up, it’ll spew a plume of debris and change course. Of course, anyone who’s ever tried to set a ball of paper aflame with a magnifying glass knows it’s not easy. Now imagine that the ball was rotating and traveling faster than sound.

Electric propulsion.To adjust course, land a spacecraft on an asteroid, fire up some rockets and push off. It’s a potentially powerful approach, but controlling that push on a spinning rock will be difficult.

Gravity tractor (pictured above). Every object exerts a gravitational pull, including a single spacecraft. Merely by hovering above the asteroid, it could pull the rock off course. The approach could even be tried with the asteroid belt-exploring Dawn spacecraft, scheduled to finish its tasks by 2015. This is potentially much easier than electric propulsion, but not quite as powerful.

Solar sails. Installing a photon-catching sail on an asteroid would be even harder than landing a ship, but it would certainly be prettier.

None of these approaches will work unless people see the asteroid in time to plan for it.

Under NASA’s Near Earth Objects program, six U.S. observatories
"search every clear night for these kinds of objects. They are tracked, cataloged and stored," said Steve Chesley, an astronomer at NASA’s Jet
Propulsion Laboratory. "NASA’s goal is to find 90 percent of those that are one kilometer across and larger. We’re at 82 percent right now, and we’ve only been aggressively searching at current levels for eight to 10
years. Those ones just haven’t flown into view."

Chesley declined to comment on the program’s budgetary status, but other astronomers have called for an expansion of its shoestring $4.1
million budget. Congress asked NASA in 2005 to increase its survey efforts, but then-agency director Michael Griffin refused to divert the estimated $1 billion needed for an overhaul away from other projects.

"A survey isn’t something you can do just once and close the book and walk away. Even if you’ve discovered an asteroid, you can only predict out so far … a couple hundred years into the future at most. The asteroids need to continue to be observed,"
said Chesley. "Without early discovery, there are no options. All the deflection technology in the world will not save you if you haven’t discovered the asteroid before it comes to you."